In order to investigate the effects of the dietary supplementation of organic acids, prebiotics and probiotics on broiler chickens, a total of 288 day-old male Cobb chicks were allocated in a completely randomized design according to a 2 x 4 factorial, consisting of two levels of crude protein (recommended or low (85% of recommended)) and a four feed-additive programme. The basal diet without any feed additive served as control and one of the following additives, organic acids, a prebiotic and a probiotic, were added to the basal diet to form the other treatments. Starter and finisher diets were offered from 1 to 21 d and 22 to 42 d of age, respectively. The birds were reared in an open-sided house system under natural tropical conditions with the same housing and general management practices. Lowering the dietary protein level significantly decreased bird performance throughout the experiment. Additives had no significant effects on body weight, body weight gain and feed intake. Dietary supplementation with the prebiotic resulted in significant improvements in feed efficiency during 22 - 42 d and 1 - 42 d of age. At 21 and 42 d of age birds fed the diets containing the prebiotic had the longest duodenal villi compared to the other treatments. Jejunum villi, and duodenum and jejunum crypt depth were not influenced by additives. At 21 d of age the dietary addition of the prebiotic and organic acids significantly increased the antibody titres against Newcastle disease compared to the control group. It could be concluded that under the condition of the current study, the prebiotic affected performance, small intestinal morphology and immunity of broiler chickens significantly.